Angle steels or angles have been frequently used as a member of the structure for many years.
FIG. 1 is a view showing one example of the cross section of an angle produced by hot-rolling, in which reference character A represents the length of one side of a flange (A being hereinafter referred to as "flange"), t the thickness of the flange, r.sub.1 the inside radius of an angled corner portion, and r.sub.2 the outside radius of the corner portion. In general, these dimensions are stipulated by JIS (Japanese Industrial Standards) except for the outside radius of the corner portion. Those angles which are made of stainless steel for use in kitchens or chemical plants require an angled corner portion having an outside radius r.sub.2 not exceeding 1 mm (hereinafter simply referred to as "sharp edge") to secure the aesthetical appearance of a structure which is built up with the angles.
The method of producing angles is generally divided into two types; the first method is a hot-rolling method using a mill having grooved rolls between which a continuous cast bloom is passed, the other is a method of producing a light angle from a steel strip by means of a roll forming (bending) machine while the steel strip is still in hot or cold state.
The term "steel strip" is used herein to refer to a narrow band-like sheet material produced by slitting a wide steel sheet in the longitudinal direction.
FIG. 2 is a view showing a succession of passes (pass schedule) defined between two rolls for producing an angle by the hot-rolling method. In the production of the angle, a continuous cast bloom (billet) used as a blank is hot-rolled into the angled shape shown in FIG. 1 by making seven, eight or more passes between seven or eight pairs of grooved rolls.
In such rolling, when a material M' to be rolled passes between the grooved rolls, friction is generated due to a difference in peripheral speed between the grooved rolls at respective portions corresponding to flanges of an angle. However, the friction thus generated deteriorates surface qualities of the angle. To deal with this problem, an improved method of producing a stainless steel angle of excellent surface qualities has been proposed as disclosed in Japanese Patent Laid-open Publication No. 5-237503, which includes forming rolls disposed in front of the grooved rolls to bend a corner portion of the material.
FIG. 3 is a view showing a method of producing an angle from a steel strip via a cold-forming process. In this method, a blank sheet S is worked or processed by bending, and hence is not subjected to a reduction in thickness. Accordingly, the outside radius r.sub.2 of an angled corner portion is about twice the thickness of the blank sheet. For example, a stainless steel strip of 3 mm in thickness is formed into an angle having a flange thickness of 3 mm, a corner portion of the angle has an outside radius r.sub.2 of about 6 mm. The angle thus produced is not suitable for use in the kitchen.
When a blank sheet composed of a steel strip is to be shaped by hot rolling into an angle, the thickness B of a corner portion of the angle shown in FIG. 1 requires to be about 1.5 times the flange thickness t. This means that the use of a steel strip having the same thickness as the flanges of a rolled product is unable to realize rolling of an angle having a desired sharp edge. To realize the desired rolling, a steel strip having a thickness greater than the thickness of the corner portion should be used.
FIG. 4 is a view illustrative of the manner in which a rough-rolled material for an angle is produced from a steel strip having a thickness greater than the thickness of a corner portion of the angle, in which FIG. 4(a) is a view showing the cross section of the rough-rolled material, and FIG. 4(b) is a view showing edge waves appearing on the rough-rolled material. As shown in FIG. 4(a), by using a two high mill, the steel strip S (indicated by the broken line) of a thickness T greater than the thickness B of a corner portion of the angle as in FIG. 1 is shaped into the rough rolled material M" by reducing the thickness of the steel strip S at portions C corresponding to flanges of an angle.
However, since the thickness of the steel strip is much smaller than the width, when opposite sides of the steel strip being rolled undergo a great reduction in thickness except a portion (a central portion, for example), wave-like wrinkles, called "edge waves", such as shown in FIG. 4(b) are generated due to variations in widthwise elongation or spread of the steel strip. The edge waves may still present as wave-like deformations on the flanges of a finished angle even when finish rolling is completed. As a result, the angle is evaluated as a defective product.
The rolling method of shaping an angle by hot rolling with the use of a rolling apparatus or system having grooved rolls defining multiple passes as shown in FIG. 2 is not suitable for the production small-sized angles made of stainless steel. For instance, when an angle having flanges of 30 mm in length and 3 mm in thickness (the size of such angle being hereinafter referred to as 30.times.30.times.3) is to be produced from a continuous cast billet having a 120 mm square cross section, the billet requires to be passed through the passes more than 15 times (15 passes). During that time, the roll-finishing temperature drops below 800.degree. C. at which the material has an insufficient degree of workability, resulting in a product having deteriorated surface qualities. To deal with this problem, reheating must be incorporated during the rolling, which will, however, incur a reduction of the rolling efficiency.
According to the method in which a hot or a cold blank sheet consisting of a steel strip is bent, production of an angle having a sharp edge is not possible, as discussed above. Further, the method of reducing a steel strip only at a portion corresponding to the flanges of an angle to be produced encounters a problem that edge waves are generated when a severe reduction is employed. To deal with this problem, a small reduction must be employed, however, this will result in an increased number of rolling passes required and an increased equipment cost. Yet, the method including the use of forming rolls disposed in front of the grooved rolls creates a problem that due to a reduction in thickness of the angled corner portion, a sharp edge is difficult to obtain. Another problem is that this rolling system is complicated as a whole.
As discussed above, an angle having a sharp edge can be produced from a cast billet by the use of a rolling system having grooved rolls.
However, so far as a small-sized angle is concerned, such rolling system requires an increased number of rolling passes and tends to deteriorate the surface qualities of the angle.